Bacillus thuringiensis (Bt) crystal (Cry) proteins have been used for decades to kill insect pests, including mosquitoes and black flies that transmit human diseases. Cry proteins related to those that kill insects have now been discovered to kill nematodes, although they are not currently used for nematode control. Cry proteins are non-toxic to mammals, making them excellent choices as insecticides and potentially as nematicides. Nematicidal Cry proteins intoxicate a wide range of nematodes, including C. elegans and two mammalian parasitic nematodes that infect the intestine. Intestinal nematodes infect over 1/4 of the human population making them an important medical target for Cry proteins. This application proposes to take advantage of the excellent tools available with C. elegans to study in depth how Cry proteins function, how resistance to Cry proteins develop (an important question even for insects), and, for the first time, address whether Cry proteins have utility in controlling mammalian parasitic nematodes. Variants of one Cry protein with better toxicity towards nematodes will be evolved and then tested for altered functions to correlate structure of toxin with function of toxin (Specific Aim I). This application also proposes to explore the mechanism of resistance to a different nematicidal Cry protein in C. elegans and determine whether this nematicidal protein may be an excellent companion therapeutic for other Cry proteins (Specific Aim II). The information and Cry protein variants generated and studied in Specific Aims I and II will then be applied towards therapy of mammalian parasites. Cry proteins, wild type and improved variants, will be orally administered to rodents infected with intestinal parasites (including one that can infect humans) to optimize therapeutic regimens and to determine which Cry proteins can have a therapeutic effect against the nematodes. The goal of these trials is to lay a foundation for future human studies.